The field of the invention is magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) methods and systems and more particularly a method and apparatus for rapidly acquiring MRI data from a portion of an imaged object.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0) the individual nuclei in the tissue attempt to align their magnetic moments with this polarizing field but as a result of nuclear spin, precess about it in random order at their characteristic Larmor frequency. The Larmor frequency is dependent on the strength of the magnetic field and on the properties of a particular nucleus as represented by a magnetogyric constant xcex3. Nuclei which exhibit this phenomenon are referred to as xe2x80x9cspinsxe2x80x9d.
By convention, the polarizing field B0 is considered to lie along a z axis of a Cartesian coordinate system. The procession of the nuclei in the polarizing field B0 creates a net magnetic moment Mz in the direction of the polarizing field. Individual spins have magnetic moments that are perpendicular to the z axis in the transverse or x-y plane, however, the random orientation of the spins cancels any net transverse magnetic moment.
In MRI imaging, a radio frequency signal is applied in the x-y plane near the Larmor frequency to tip the net magnetic moment into the x-y plane so that it rotates at the Larmor frequency. The practical value of this phenomenon resides in the signal which is then emitted by the excited spins termed the NMR signal (xe2x80x9cnuclear magnetic resonancexe2x80x9d). In simple systems, the excited spins induce an oscillating sine wave in a receiving coil which may be the same coil used to excite the spins. The amplitude of this signal decays as a function of the homogeneity of the magnetic field caused by atomic scale interaction between the spins or xe2x80x9cspin-spinxe2x80x9d relaxation and the engineering limitations of producing a truly homogenous polarizing field B0. This decay is caused by a loss of phase coherence in the spins and is commonly referred to as T*2 relaxation. Second decay mechanism is the gradual return of the magnetic moments of the individual spins to a longitudinal direction aligned with the polarizing field B0. This is termed T1 relaxation and in most substances of medical interest is much longer than T2 relaxation.
An image of a patient may be obtained by evaluating the NMR signal contributed by different spins at different locations in the patient""s tissue. A pulse sequence using gradient magnetic fields encodes location information on the spins in the form of the phase and frequency. The encoded spin signal may then be separated to produce an image.
A wide variety of pulse sequences is known. For example, the spin warp or spin echo technique is described in xe2x80x9cSpin Warp NMR Imaging And Applications To Human Whole-Body Imagingxe2x80x9d by W. A. Edelstein et al., Physics in Medicine and Biology, vol. 25, pp. 751-756 (1980); the steady state free precession (xe2x80x9cSSFPxe2x80x9d) technique including gradient refocused acquired steady state pulse sequences (xe2x80x9cGRASSxe2x80x9d) as described in U.S. Pat. No. 4,665,365 and contrast enhanced fast imaging (SSP-ECHO) described in xe2x80x9cRapid Fourier Imaging Using Steady State Free Precisionxe2x80x9d, R. C. Hawks and S. Patz, Magnetic Resonance in Medicine 4, pp. 9-23 (1987); and echo planer imaging (xe2x80x9cEPIxe2x80x9d) is described in an article by Peter Mansfield (J. Phys. C. 10: L55-L58, 1977). These descriptions of pulse sequences are hereby incorporated by reference.
In a representative spin echo pulse sequence, a z-axis gradient and a narrow-band, radio frequency excitation pulse may be applied to a patient, for example, so that only spins in a xe2x80x9cslicexe2x80x9d to the patient perpendicular to the z-axis are excited. An x-gradient field may then be applied to cause the spins at one side of the slice to precess faster than spins on the other side of the slice. In this manner, the spins have been given a frequency encoding that allows them to be distinguished along the x-axis.
This NMR signal of the spins at different frequencies is acquired for a period of time and digitized to provide a first row of data that may be stored in an array in a reconstruction computer. The number of dimensions of the array and the number of elements in the array define a k-space well known to those in the art. The NMR signals must be sampled at a rate at least twice the frequency of the highest frequency component of the NMR signal (the Nyquist sampling rate) so as to prevent the introduction of aliasing artifacts.
Additional NMR signals are then collected for this slice with the same x-gradient but with a progressively increased y-axis gradient field. The y-axis gradient serves to phase encode the spins in the y-direction. Each successive NMR acquisition with a different y-axis gradient forms a successive row in the k-space array in the computer.
Once k-space has been filled, a two dimensional Fourier transform may be made of the k-space data to produce the desired image. Generally it is known to band limit the NMR signal to eliminate the contribution of spins beyond certain spatial ranges in the frequency encoding x-axis direction limiting the amount of k-space data somewhat. Such band limiting cannot be performed in the phase encoding direction, however, until after the data is fully acquired and therefore is of little value in reducing the data acquisition time.
In an alternative method of data acquisition, the k-space data is filled not by rows and columns but by a series of radial projections about a point within k-space. This acquisition technique is analogous to the acquisition of data in an x-ray computed tomography (xe2x80x9cCTxe2x80x9d) machine and allows the data to be reconstructed into an image by CT-type algorithms including filter back projection.
In MRI angiography, images of the blood vessels are obtained. For contrast-enhanced applications in which contrast materials such as gadolinium compounds are injected into a peripheral vein, the acquisition of k-space data must be carefully coordinated with the arrival of contrast so as to prevent an unfavorable variation in the weighting of the k-space data. The availability of a high speed imaging technique would be helpful in this regard since it would permit a series of images to be obtained throughout the passage of contrast.
In contrast enhanced MRI, two images, one before the introduction of a contrast medium into the vessels and one after the introduction of the contrast medium, may be obtained and subtracted. The subtracted image reveals information about the bloodflow through the vessels allowing the detection of obstructions and the like. Structures other than flowing blood are similar in the two images and thus substantially reduced in contrast.
The timing of the acquisitions of the two MRI images is crucial to providing a high contrast image. Normally there is a time delay between the introduction of the contrast medium into the patient and its time of arrival at the region of the vessel of interest.
Ideally the first image should be concluded immediately before the arrival of the contrast medium so as to provide an accurate comparison image and the second image begun immediately after the arrival of the contrast medium so as to be complete before the contrast medium dissipates The time consuming process of acquiring an image of a patient and the difficulty of monitoring the progress of the contrast medium, make production of a high quality contrast enhanced MRI image a difficult task.
It would be advantageous to be able to acquire images of higher resolution more quickly. This is important when the available imaging time is limited by the passage of injected contrast material or by respiratory motion. In Cartesian acquisitions image spatial resolution is proportional to imaging time, so any reduction in acquisition time produces images of reduced spatial resolution. Within the context of Cartesian imaging, some investigators have developed methods to image reduced field of view. Reduction of the field of view reduces the amount of data required. Therefore, imaging time can be reduced. A disadvantage of this is that in Cartesian imaging objects from outside the field of view can appear inside the selected small field of view due to aliasing. In Cartesian imaging this aliasing results in an artifact which looks exactly like the object from outside the field of view. Although band limiting can be used to reject objects from outside the field of view in the frequency direction, this is not possible in the phase encoding direction because each row of phase encoding data relates to spins situated throughout the field of view.
One method of contending with this problem is the method of Hu and Parrish (X. Hu, T. Parrish, Reduction of FOV for dynamic imaging. Magn. Reson. Med. 31, 691 (1994). In this method, a preliminary image of the entire field of view is obtained. The portion of this image corresponding to the desired field of view is set to zero. Then the remaining data are subjected to a Fourier transformation to provide a k-space data set corresponding to just that material which might alias into the small field of view. During dynamic imaging of the small field of view, this k-space data set is subtracted from the data associated with the small field of view. This removes aliased signals from the small field of view, but only if the objects outside the small field of view are truly stationary.
In projection MRI acquisitions there is no phase encoding direction, as such, and it has therefore been proposed to use such an acquisition to limit the field of view thereby reducing acquisition time. Each projection is bandlimited to limit the inclusion of spins from outside the region of interest along the projection, however spins in regions perpendicular to the axis of projection cannot be so eliminated and produce image artifacts. These artifacts must be addressed by estimation techniques or supplementary measurements of the out of region areas for later cancellation. Such a technique is described in xe2x80x9cZooming by Back Projectionxe2x80x9d by K. Scheffler in the Proceedings of ISMRM, Fifth Scientific Meeting and Exhibition, Vancouver. BC Canada, Volume 1 page 288. Scheffler and Hennig (K. Scheffler, J. Hennig, Reduced circular field-of-view imaging. Magn. Reson. Med. 40, 474-480 (1998).) have applied the Hu/Parrish algorithm using projection acquisition. In this case, as in the Cartesian case, the imaging speed for a given resolution is increased. However, dynamic changes can only be viewed within the reduced field of view, and aliasing occurs if the outer material is not stationary. These techniques are limited by the fact that only a small field of view is acquired.
In projection imaging, the requirement that the angular sampling interval be equal to the radial sampling interval is that the number of projections NP is related to the number of radial samples NR by
NP=NRxc2x7xcfx80/2
This requirement poses an inherent time disadvantage for projection imaging relative to Cartesian imaging. The fact that it requires an additional factor of xcfx80/2 to acquire an image of the same resolution is one of the reasons that Cartesian acquisition is the primary method for magnetic resonance imaging in spite of the fact that the first MRI images in the mid 1970s were acquired with projection acquisition.
It is well known in the X-ray computed tomography literature, where projection acquisition is used, that acquisition time can be increased if sparse angular sampling is used. In this method the number of projections is decreased. It is also well known that as the number of angular samples is decreased, spatial resolution is not decreased but radial streak artifacts emanate from all objects within the field of view. In X-ray CT where bone provides a dominant signal which far exceeds the tissue signal differences to be distinguished, the presence of such artifacts is completely unacceptable. Up until this point in time, it has been assumed by the MRI community that such artifacts would be similarly unacceptable in magnetic resonance imaging if sparse angular sampling were to be used. Therefore, the small field of view techniques mentioned above have been resorted to in order to increase imaging speed in the limited number of situations in which the field of view can be compromised.
The present inventors have recognized that when the NMR data is acquired in projection rather than Cartesian fashion the rate at which spatial resolution can be acquired is significantly increased. As can be inferred from X-ray computed tomography, spatial resolution is completely determined by the resolution in the readout direction within each projection and not by the number of acquired projections. As the number of projections is decreased, resolution is unaffected. The only effect is an increase in artifacts The artifacts generated by the sparse sampling in projection imaging are different from those generated in Cartesian acquisitions and can be readily tolerated in a number of important imaging applications.
Around each object within the overall field of view there is a small region (local field of view) in which the object does not produce any artifacts. The size of this artifact-free region does depend on the number of acquired projections. Outside of this small region each object does produce streak artifacts which can enter the local fields of view associated with other objects. The present inventors have recognized that these artifacts are typically no more than a few percent of the signal associated with the object producing them and that for applications such as angiography, pancreatography or bile duct imaging, where the signals of interest are the most dominant signals in the overall field of view, these artifacts appear to be completely tolerable. In such situations azimuthally undersampled projection imaging provides the speed and resolution advantages of reduced field of view imaging, but does so simultaneously throughout an entire large field of view. Preliminary results suggest that a speed increase of a factor of six is often possible.
Specifically, then, the present invention provides a method of MRI imaging of structures, the structures providing NMR signals with intensities dominating the intensity of other NMR signals of other materials within a field of view. The method includes the steps of generating a series of gradient fields along axes distributed over a range of angles about an axis in the field of view, then acquiring NMR signals of the field of view at different gradient fields, each acquisition providing an angular projection of data in k-space having data points radially spaced at distances and are along a projection. The number of acquisitions is limited to a number of projections less than NRxcfx80/4 in number (where NR is the number of radial samples) so that k-space is sparsely sampled. The projections are reconstructed to produce a field of the image of view displaying the entire field of view.
It is thus one object of the invention to provide a rapid NMR acquisition technique well suited to bright objects dominating other objects in the field of view. Such situations may include imaging blood vessels in angiography, or imaging the pancreas or liver ducts. The present invention provides a dynamic view of the entire field of view, not just the object.
The projection acquisition may be combined with a volumetric acquisition in which a phase encoding in gradient is applied along the axis during the acquisition of the NMR signals and a Fourier reconstruction is made along that axis to provide a volume image.
Thus it is another object of the invention to provide for the benefits of projection acquisition together with the benefits resulting from weighted k-space acquisition along the phase encoding gradient. As is known in the art, it is desirable to increase k-space sampling for lower k-space frequencies at the expense of higher k-space frequencies. Projection imaging does not allow such k-space weighting in the slice of the projection since each projection acquires data near the center of k-space. However, when combined with Fourier reconstruction in the volumetric axis, such weighting can be obtained. The phase encoding may be performed after a full set of projection images are obtained or may be performed in between each projection image. In this latter case, the projection angle selected may alternate between two or more interleaved sets so as to minimize the time required to obtain a sparse sampling of k-space yet to allow the more complete sampling as time permits.
In the case where the structure being imaged is smaller than a containing imaged object, the step of spatial saturation of two bands parallel to the gradient direction and separated by the intervening width of the structure may be included.
It is therefore another object of the invention to provide a method of suppressing the artifacts produced by sparse sampling with projection imaging.
During the acquisition of the NMR signals, a contrast medium may be introduced into a field of view and used to produce a contrast index indicating the arrival of a contrast medium in the structure. This contrast index may be displayed and/or used to select particular NMR signals for the generation of low and high contrast images. The low and high contrast images may be subtracted, for example, to perform subtraction angiography.
Thus it is another object of the invention to provide a real-time measure of contrast. The NMR signals acquired at each projection may be simply integrated to produce a contrast indication. The contrast indication may be weighted according to the particular angle so as to remove anatomical effects or when a comparison value is being produced, the comparison may be made only between corresponding angles.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.